1. Technical Field
The present invention provides a catalyst system for the polymerization of olefins that include the combination of a conventional supported Ziegler-Natta catalyst with a mixture of different electron donor compounds, both having the general formula SiR.sub.m (OR').sub.4-m where R is selected from the group consisting of an alkyl group, a cycloalkyl group, an aryl group and a vinyl group; R' is an alkyl group; and m is 0-3, wherein when R is an alkyl group, R may be identical with R'; when m is 0, 1 or 2, the R' groups may be identical or different; and when m is 1, 2 or 3, the R groups may be identical or different. Preferably, one electron donor is of the formula: ##STR3## wherein R.sub.1 and R.sub.4 are both an alkyl or cycloalkyl group containing a secondary or tertiary carbon atom attached to the silicon atom, R.sub.1 and R.sub.4 being the same; R.sub.2 and R.sub.3 are alkyl or aryl groups, R.sub.2 and R.sub.3 being the same or different;
and the other is of the formula: ##STR4## wherein R'.sub.1 is an alkyl or cycloalkyl group containing a primary, secondary or tertiary carbon atom attached to the silicon atom, R'.sub.2 and R'.sub.3 are an alkyl or aryl group, R'.sub.2 and R'.sub.3 being the same or different; and R'.sub.4 is an alkyl group with a primary carbon attached to the silicon atom, R'.sub.1 and R'.sub.4 being the same or different.
2. Description of Prior Art
Catalyst systems for the polymerization of olefins are well known in the art. Typically, these systems include a Ziegler-Natta type polymerization catalyst; a co-catalyst, usually an organoaluminum compound; and an electron donor compound, usually an organosilicon compound. Examples of such catalyst systems are shown in the following U.S. Pat. Nos.: 4,107,413; 4,294,721; 4,439,540; 4,115,319; 4,220,554; 4,460,701; and 4,562,173; the disclosures of these patents are hereby incorporated by reference. These are just a few of the scores of issued patents relating to catalysts and catalyst systems designed primarily for the polymerization of propylene and ethylene.
A Ziegler-Natta type polymerization catalyst is basically a complex derived from a halide of a transition metal, for example, titanium, chromium or vanadium with a metal hydride and/or a metal alkyl that is typically an organoaluminum compound. The catalyst is usually comprised of a titanium halide supported on a magnesium compound complexed with an alkylaluminum.
The development of these polymerization catalysts has proceeded seemingly in generations of catalysts. The catalyst disclosed in the patents referenced above are considered by most to be third generation catalysts. With each new generation of catalysts, the catalyst properties have improved. Particularly, the efficiencies of the catalysts, as expressed in kilograms of polymer product per gram of catalyst in two hours, have increased from the 1-3 range to the 10-12 range and beyond. Catalysts have been developed that not only have higher efficiencies but also retain their activity over a longer period of time, thus being able to produce more polymer product over the life of the catalyst. Any increase in the efficiency and life of the catalyst leads to lower costs due to lower catalyst consumption, and it also lowers the capital expenditures in building and operating a plant as the size of the reactors are lowered for specific plant capacities and the reactor residence time is lowered. A higher efficiency also leads to a cleaner polymer product thereby avoiding the need to wash or treat the product to remove catalyst ash residuals.
In addition to the improved catalysts, improved activation methods have also lead to increases in the catalyst efficiency. A most recent discovery includes a process for pre-polymerizing the catalyst just prior to introducing the catalyst into the reaction zone. This process is disclosed in U.S. Pat. No. 4,767,735 the disclosure of which is hereby incorporated by reference.
In addition to the development of new catalysts and new reaction processes, a discovery of a more appropriate co-catalyst or electron donor to go with the new generation of catalysts in forming a total catalyst system would be of great benefit to the polymerization art especially if it lead to improved stereoselectivity of the polymer product. In such a total catalyst system, a co-catalyst activates the catalyst and provides the initiation of a polymer chain. The co-catalyst that works well with the new generation catalysts is an organoaluminum compound, most typically triethylaluminum (TEAI) or another trialkylaluminum. Examples of other useful organoaluminum compounds include an alkylaluminum dihalide, a trialkoxyaluminum, a dialkylaluminum halide, and a triisobutylaluminum.
An electron donor compound is used in the polymerization reaction to reduce the atactic form of the polymer thereby giving control of and increasing the production of isotactic polymers. Although a broad range of compounds are known generally as electron donors, a particular catalyst may have a specific compound or group of compounds with which it is especially compatible. Discovery of an appropriate type of electron donor which gives significant increases in control of the isotactic index of the desired polymer product and other properties of the product would be highly advantageous.
The present invention comprises such a discovery. It has been surprisingly discovered that a combination of two specific groups of silane compounds serving as electron donors in combination with a conventional supported Ziegler-Natta type of catalyst results in significant increases in stereoselectivity over that for this particular type of catalyst using one of these electron donors alone.